1. Field of the Invention
The present invention relates to novel compositions and methods for bleaching metals in photographic materials. More particularly, the present invention relates to novel bleaching agents and their use in aqueous and nonaqueous photographic materials. One of its aspects relates to novel bleaching solutions which are capable of bleaching preformed or developed silver in aqueous and nonaqueous photographic materials. Another of its aspects relates to novel bleaching agents for incorporation into radiation sensitive elements, particularly photothermographic elements, for converting metallic image silver in said elements to a colorless, innocuous product.
2. Description of the Prior Art
In the processing of photographic silver halide color elements and in some black-and-white photographic processes, it is necessary to remove metallic silver formed during development. This is ordinarily accomplished by the use of a silver bleaching composition which converts metallic silver into a soluble silver salt. A bleach commonly used for this purpose is an aqueous solution of a water-soluble ferricyanide, such as sodium or potassium ferricyanide, and an alkali metal bromide. Aqueous bleaching solutions comprising aromatic nitroso bleaching agents are described in U.S. Pat. Nos. 2,625,477 (Sawdey), issued Jan. 13, 1953 and 2,705,201 (Tulagin), issued Mar. 29, 1955. U.S. Pat. No. 3,707,374 (Van Der Voorn et al), issued Dec. 26, 1972, relates to aqueous bleaching solutions comprising water-soluble persulfate salts or water-soluble nitrosodisulfonate salts and aromatic amines.
Combined bleaching and fixing compositions for use in photography, commonly referred to as bleach-fix compositions have been known for many years. They are used in processing silver halide photographic materials to simultaneously accomplish the steps of bleaching and fixing and thereby eliminate one step in the conventional processing procedure. The essential components of a bleach-fix composition are the bleaching agent, i.e. an agent which oxidizes the metallic silver in the silver image to a soluble form, and the fixing agent, i.e., an agent which dissolves the undeveloped silver halide and the silver salts formed by the action of the bleaching agent. U.S. Pat. No. 3,189,452 (Bard et al), issued June 15, 1965, describes aqueous bleach-fix compositions prepared by combining conventional bleaching agents such as alkali metal dichromates and alkali metal ferricyanides with conventional fixing agents such as alkali metal thiocyanates. Other known aqueous bleach-fix solutions include those described in U.S. Pat. No. 3,615,508 (Stephen et al), issued Oct. 26, 1971; U.S. Pat. No. 3,634,088 (Cooley), issued Jan. 11, 1972, wherein the solutions comprise a ferric salt of an aminopolycarboxylic acid as a bleaching agent and a thiosulfate as a fixing agent; U.S. Pat. No. 3,706,561 (Mowrey et al), issued Dec. 19, 1972, wherein ferric ion-ligand complexes are used as bleaching agents and thiosulfates, thiocyanates, thioethers, or thioureas are used as fixing agents; and U.S. Patent application Ser. No. 389,063 (Barton et al), filed Aug. 16, 1973 now U.S. Pat. No. 4,033,771.
U.S. Pat. Nos. 3,600,168 and 3,600,169 (Lawton), both issued Aug. 17, 1971 disclose the use of stable free radicals, such as nitroxyls, in compositions for electrostatic light sensitive reproduction sheets. Chemical Abstracts, Vol. 64, 1966, Abstract 198876 discloses the use of free radical nitroxyl compounds as antioxidants and ultraviolet light absorbers in polymeric materials. U.S. Pat. No. 3,322,542 (Ullman et al), issued May 30, 1967, teaches the use of nitroxides to stabilize polychromic compounds against molecular deterioration.
British Pat. No. 1,326,889 (Konishiroku), published Aug. 15, 1973 teaches the use of nitroxyl radicals as antifading agents in conventional color photographic materials. Konishiroku, however, does not teach or suggest the use of stable free radical nitroxyl compounds as bleaching agents in bleaching solutions and radiation sensitive elements, particularly photothermographic elements.
Copending U.S. application Ser. No. 367,304, filed June 5, 1973 by Chang et al now abandoned teaches the use of nitroxyl radicals as selective oxidizing agents in photographic elements such as color diffusion transfer elements.
It is well known to develop a latent image in a photothermographic element using thermal processing. After imagewise exposure, the resulting latent image in the photothermographic element is developed and, in some cases, stabilized, merely by uniformly heating the photothermographic element. Such materials and process are described, for example, in U.S. Pat. Nos. 3,152,904 (Sorensen et al), issued Oct. 13, 1964, 3,301,678 (Humphlett et al), issued Jan. 31, 1967; 3,392,020 (Yutzy et al), issued July 9, 1968; 3,457,075 (Morgan et al), issued July 22, 1969; British Pat. No. 1,131,108 published Oct. 23, 1968; German Pat. No. 888,045 issued June 29, 1943 and British Pat. No. 1,161,777 published Aug. 20, 1969. Certain photothermographic materials for producing an image in color are also known as described, for example, in U.S. Pat. No. 3,531,286 (Renfrew), issued Sept. 29, 1970 and U.S. Pat. No. 3,761,270 (deMauriac et al), issued Sept. 25, 1973.
U.S. Pat. No. 4,021,240 of Cerguone et al, issued May 3, 1977, relates to photothermographic and thermographic elements, compositions and processes for providing a developed image in color. There is no teaching or suggestion in this application of the present invention.
Copending U.S. application Ser. No. 662,403 of Mowrey and Oftedahl, filed Mar. 1, 1976, now abandoned relates to an activator sheet for a dry thermal silver dye bleach process. The sheet comprises a support, a diffusible mineral or organic acid and a nonhydrolyzable polymeric vehicle. This application does not teach or suggest the present invention. There is no prior art which teaches or suggests the present invention. This invention provides novel and unobvious bleaching solutions and radiation sensitive elements, particularly photothermographic elements, comprising stable free radical nitroxyl bleaching agents. These bleaching agents provide the following advantages over bleaching agents in the prior art:
1. they may be incorporated in photographic elements without adversely affecting the photographic properties of radiation sensitive emulsions, some of which may be thermally activated; PA1 2. they are capable of bleaching preformed or developed silver in both aqueous and nonaqueous photographic materials; PA1 3. they are effective in converting metallic image silver to a colorless, innocuous product by a dry thermal process; PA1 4. they are biodegradable and easily destroyed by ozonolysis, lessening environmental pollution; PA1 5. their oxidation potential and, hence, their bleaching characteristics can be controlled by the hydrogen ion concentration when they are used in bleaching solutions; and PA1 6. they are capable of acting as both a bleaching agent and a fixing agent when they comprise certain substituents. PA1 1. exposure of a photothermographic element to light, said element comprising a support having thereon a layer comprising a reducing agent, a silver salt oxidizing agent and a silver halide; and PA1 2. thermal development at a temperature above about 80.degree. C., an improvement of which process comprises the incorporation into the photothermographic element, a free radical nitroxyl bleaching agent. PA1 a. a reducing agent; PA1 b. a silver salt oxidizing agent; and PA1 c. a silver halide; and in the same or a different layer, a stable free radical nitroxyl bleaching agent having a formula selected from the group consisting of: ##STR12## wherein Z, y, m, n, p, R.sup.1, R.sup.2, R.sup.3, R.sup.4, R.sup.5, R.sup.6, L and L' are as defined hereinabove in the description of nitroxyl bleaching agents for photographic bleaching solutions. PA1 2-(3,5-di-tert-butyl-4-hydroxyphenyl)-4,5-diphenylimidazole, PA1 2-(4-hydroxy-3,5-dimethoxy)-4,5-bis(p-methoxyphenyl)imidazole, and bis-(3,5-di-tert-butyl-4-hydroxyphenyl)phenylmethane. PA1 (1) exposure of a photothermographic element to light, said element comprising a support having thereon a layer comprising a reducing agent, a silver salt oxidizing agent, and a silver halide; and PA1 (2) thermal development at a temperature above about 80.degree. C.;
Nonaqueous photographic materials useful in the practice of the present invention include those in which a radiation sensitive element is contained in a water-insoluble or a water-impermeable binder. Exemplary of these materials are photothermographic elements adapted for thermal development which contain poly(vinyl butyral) or other suitable binders.
Aqueous photographic materials include those in which a radiation sensitive element is contained in a water-soluble or a water-permeable binder, such as hydrophilic proteinaceous colloids like gelatin. Exemplary of these materials are conventional silver halide photographic elements.